This invention relates to an electrode assembly for gas-forming electrolyzers, particularly for monopolar membrane electrolyzers comprising vertical plate electrodes and opposite electrodes and a membrane between the plate electrode and the opposite electrode.
In electrochemical processes it is important to achieve a uniform distribution of the current over the surface of the electrodes. The uniform distribution will depend on the throwing power of the electrolyte and on the homogeneity of the electrode. Whereas an inadequate throwing power can be compensated for by an increase of the interelectrode distance, this will increase the voltage drop across the cell. If the surface of the electrode is nonhomogeneous, the flow of current will result in local distortion. For this reason it is important to provide a uniform distance between the anode and cathode. In membrane electrolytic cells used for the commercial production of gases, such as chlorine, oxygen and hydrogen, the adjustment and maintenance of hydrogen and the adjustment and maintenance of a defined interelectrode distance involves a very high expenditure. If the interelectrode distance is too small, the gas bubbles cannot escape as quickly as is required. If the distance is large, the gas bubbles will escape quickly but the voltage across the cell will be higher owing to the higher resistance of the electrolyte. Cells are often proposed in which the interelectrode distance equals zero because the active anode structure and the anode/cathode structure are in direct contact with the membrane. In such cells, the membrane will have a shorter life because local current peaks cannot be avoided.
A presence of gas in the electrolyte between the electrodes will reduce the electrical conductivity of the electrolyte and will thus increase the energy consumption. The presence of such gas may also result in current-induced microdistortions on the surface of the electrodes. The evolution of gas will give rise to turbulence in the electrolyte. A turbulence in the electrolyte is undesirable because it will cause the membrane to be subjected to intense mechanical loads. To avoid a mechanical destruction of the membrane, it is generally necessary to limit the height of the electrodes, to provide a substantial distance between the electrodes in the cell, and to limit the electric current density, although this will reduce the energy efficiency of the electrolytic cell and its productivity.
In order to avoid the disadvantages of electrolytic cells comprising membranes and vertical electrodes, it is common to use apertured electrodes, i.e., electrodes having openings for the escape of the gases produced by the reaction. Such electrodes may consist, e.g., of perforated electrodes, woven wire mesh or expanded metal. The use of such electrodes will result in disadvantages residing, i.e., in a smaller active surface area, in inadequate mechanical stability and a loss of high-quality coating material on the rear side of the electrodes.
It is known from German Patent Publication No. 20 59 868 that gas-forming diaphragm cells comprising vertical electrodes may be provided with a plate electrode consisting of individual plates, which have surfaces for guiding the gas which has been produced and is to be removed. In the electrolyzer known from French Patent Specification No. 10 28 153, the electrodes are parallel to each other and have the smallest possible spacing. The known electrodes each consist of one plate or a plurality of plates. The plates have horizontal slots, which are defined by edge flanges of the plate strips and present the smallest possible resistance to the escape of gas. The edge flanges are directed toward the opposite electrode and the active surface area is not substantially decreased.
Published European Patent Application No. 102,099 discloses an electrode assembly for gas-producing electrolyzers comprising electrode plates which are divided along a plurality of continuous horizontal lines. A certain geometry has been adopted to promote the escape of gas from the electrolyte.
The electrodes of electrolytic cells are ideally used also to conduct electric current. That use will not give rise to problems in bipolar cells, where the current flows through the electrode in the direction of the electrolysis current so that an adequate cross-sectional area for the flow of the current will always be available. On the other hand, in monopolar cells the current in the electrode must flow transversely to the electrolysis current. Whereas surface electrodes can be used for that purpose, it is not possible to readily use wire netting and expanded metal, particularly in electrolytic cells which differ from diaphragm cells in that they operate at current densities above 3 kA/m.sup.2. In that case it is usual to conduct the current by internal elements, such as conductor rods, from which the current is distributed over the active surfaces of the electrodes (Published German Application No. 28 21 984).
In the electrolysis of aqueous solutions of alkali chloride by a membrane process using non-selective membranes, the ion-selective membrane contacts the anode sheet structures owing to the different densities of the alkali hydroxide in the cathode compartment and the acid aqueous alkali chloride solution in the anode compartment. Because the electrolyte is absent from or present only in a very small quantity at said contact surface of the membrane, no electrolysis or only a very weak electrolysis can take place at said contact surface. For this purpose, expanded metal, perforated plates or similar electrode plates of titanium are used for commercial electrolysis so that an electrolysis can take place at the edges of the holes or of the expanded metal and in part also on the rear of the electrode plates. But this involves a loss of active electrode surface area so that an undesirable voltage rise results.